The output current of a switching mode power supply can influence the performance of a system, e.g., the brightness of an LED driven by the power supply. Thus, accurate control of the average output current is rather important. FIG. 1 is a prior art switching mode power supply 100 with average current control. As shown in FIG. 1, the switching mode power supply 100 is a flyback converter that receives an AC input signal and provides an output voltage to a load, e.g., LEDs. The switching mode power supply 100 includes a rectifier bridge 101, a transformer 102, a zero-crossing detector 103, an isolated feedback circuit 104, a controller 105, a switching circuit 106, a primary current sense resistor 107-1, and a secondary current sense resistor 107-2. The transformer 101 comprises a primary winding 102-1, a secondary winding 102-2, and an auxiliary winding 102-3. The switching circuit 106 comprises a switch. The switching mode power supply 100 further includes an input capacitor (CIN) coupled across the rectifier bridge 101, a diode 108 coupled in series with the secondary winding 102-2 of the transformer 102, and an output capacitor (COUT) coupled between the output port of the switching mode power supply 100 and ground.
The rectifier bridge 101 receives the AC input, and based on the AC input, provides a rectified signal to the primary winding 102-1 of the transformer 102. The primary current sense resistor 107-1 is coupled in series with the switching circuit 108 to provide a primary current signal that represents a current flow through the primary winding 102-1 of the transformer 102 to the controller 105. The secondary current sense resistor 107-2 is coupled in series with the load to provide a secondary current signal that represents a load current. The isolated feedback circuit 104 receives the secondary current signal, and based on the secondary current signal, provides a feedback signal to the controller 105. The zero-crossing detector is coupled in series with the auxiliary winding 102-3 of the transformer 102 to provide a zero detected signal to the controller 105 if a voltage zero-cross of the auxiliary winding 102-3 happens. The controller 105 provides a control signal used to toggle the switch in the switching circuit 106 in response to the primary current signal, the feedback signal, and the zero detected signal. If toggling of the switch in the switching circuit 106 is controlled, the power supplied to the secondary winding 102-2 of the transformer 102 can be adjusted, so that the average current flow through the LED is regulated.
The above control scheme requires an isolated feedback circuit for the secondary current signal, which complicates the circuit structure. In addition, an additional current sense resistor, i.e., the secondary current sense resistor 107-2 is needed, which increases power loss and reduces efficiency.